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the line AA 1 , do change in relative position. In fig. 3, as already 

 shown, the three anterior ones are on a part of the margin that is 

 directed forwards, and, there is a gradual shifting backwards of 

 these segments, with" reference to the other structures appearing on 

 the head plate. In fig. 6, there are 5 neural segments in front of 

 the region occupied by the primary (op) and the first accessory 

 (A. op 1 ) optic vesicles. I have clearly traced this latter structure 

 into the thalamencephalon , which is a part of the fore-brain, it is 

 clear, therefore, that there are at least 5 neural segments in the fore- 

 brain at this particular stage. The mid-brain (m&, in figs.' 8, 9 

 and 10) is, at this time, scarcely distinguishable; it is only in later 

 periods that it becomes expanded and arched over two neural seg- 

 ments. I shall agree, that after the head regions become established 

 and settled down to somewhat definite limits, there are then 5 neural 

 segments included in the combined fore- and mid-brain, but, there is 

 one period, when the whole 5 are included in the fore-brain, and the 

 relation pointed out by Waters is not primitive but secondary. The 

 situation presented is something like this: — The segmentation of 

 the neural axis is much older historically speaking than the division 

 of the brain into vesicles. It is a much more primitive characteristic, 

 and quite probably existed for great lapses of time before the latter 

 arose. (The order of their appearance, and the time interval between 

 the two, would indicate as much.) We thus have exhibited the re- 

 lation between two distinct morphological processes, the one, the 

 division of the embryo into segments, and the other, the modification 

 of the head-end into cranial vesicles. The former, is already in 

 existence when the latter process commences, and we have the cranial 

 vesicles superimposed upon an already segmented neural axis. It 

 seems to me, therefore, that we are not justified in going further than 

 to say, that there are a certain number of neural segments modified 

 to form the head, and, after various shiftings of position, and re- 

 arrangements of parts, there comes to be, finally, 2 of the original 

 segments included under the roof of the greatly expanded mid-brain, 

 and 3, under the dorsal wall of the fore-brain. It is not, at bottom, 

 primitive segmentation of the brain with which we are dealing, but 

 primitive segmentation of the embryo; and the anterior end of the 

 body undergoes the most extensive modifications, and departs most 

 widely from the early segmented condition. 



